In recent years, with an aim toward effective utilization of energy for greater environmental conservation and reduced usage of resources, a great deal of attention is being directed to power storage systems for electric vehicles, overnight charging electric power storage systems, and household dispersed power storage systems based on photovoltaic power generation technology.
The first requirement for such power storage systems is high energy density of the power storage elements used in them. The development of lithium ion batteries is advancing at a rapid pace, as an effective strategy for power storage elements with high energy density that can meet this requirement.
The second requirement is high output characteristics. For example, in a combination of a high efficiency engine and a power storage system (such as in a hybrid electric vehicle), or a combination of a fuel cell and a power storage system (such as in a fuel cell electric vehicle), high output discharge characteristics are required for the power storage system during acceleration.
Electrical double layer capacitors and nickel hydrogen cells are currently under development as high output power storage elements.
Electrical double layer capacitors that employ activated carbon in the electrodes have output characteristics of about 0.5 to 1 kW/L. Such electrical double layer capacitors have high durability (especially cycle characteristics and high-temperature storage characteristics), and have been considered optimal power storage elements for fields requiring the high output mentioned above. However, their low energy density of about 1 to 5 Wh/L and short output duration have been obstacles to their practical use.
On the other hand, nickel hydrogen cells employed in current hybrid electric vehicles exhibit high output equivalent to electrical double layer capacitors, and have energy density of about 160 Wh/L. Still, research is being actively pursued toward further increasing their energy density and output, further improving their stability at high temperatures, and increasing their durability.
Research is also advancing toward increased outputs for lithium ion batteries as well. For example, lithium ion batteries are being developed that yield high output exceeding 3 kW/L at 50% depth of discharge (a value representing how deeply the element is discharged). However, the energy density is 100 Wh/L or less, and the design is such that high energy density, as the major feature of a lithium ion battery, is reduced. The durability (especially cycle characteristics and high-temperature storage characteristics) is inferior to that of an electrical double layer capacitor. In order to provide practical durability for a lithium ion battery, therefore, they can only be used with a depth of discharge in a narrower range than 0 to 100%. Because the usable capacity is even lower, research is actively being pursued toward further increasing durability.
There is strong demand for implementation of power storage elements exhibiting high power density, high energy density and durability, as mentioned above, but the aforementioned existing power storage elements have their advantages and disadvantages. New power storage elements that satisfy these technical requirements are therefore desired, and power storage elements known as lithium ion capacitors are being development in recent years as promising candidates.
Lithium ion capacitors are a type of power storage element using a non-aqueous electrolyte comprising a lithium ion-containing electrolyte (or, “non-aqueous lithium-type power storage element”), wherein charge-discharge is accomplished by:
non-Faraday reaction by adsorption/desorption of anion similar to an electrical double layer capacitor, at the positive electrode, and
Faraday reaction by occlusion/release of lithium ion similar to a lithium ion battery, at the negative electrode.
An electrical double layer capacitor in which charge-discharge is accomplished by non-Faraday reaction at both the positive electrode and negative electrode has excellent output characteristics, but low energy density. On the other hand, a lithium ion battery that is a secondary battery in which charge-discharge is accomplished by Faraday reaction at both the positive electrode and negative electrode has excellent energy density but poor output characteristics. A lithium ion capacitor is a new power storage element that aims to achieve both excellent output characteristics and high energy density by accomplishing charge-discharge by non-Faraday reaction at the positive electrode and Faraday reaction at the negative electrode.
The purposes for which lithium ion capacitors are used may be electricity storage for, for example, railways, construction machines and automobiles. For such purposes, it is necessary for the capacitors used to have excellent temperature characteristics because of the harsh operating environments. In particular, performance impairment has been a problem caused by gas emissions due to decomposition of the electrolyte at high temperature. As countermeasures against this problem, there have been proposed lithium ion capacitors containing fluorinated cyclic carbonates in the electrolyte (see PTLs 1 and 2). Lithium ion capacitors containing vinylene carbonate or its derivatives in the electrolyte have also been proposed (see PTL 3). Another technology that has been proposed is that of power storage elements containing lithium bis(oxalato)borate in the electrolyte (see PTL 4). Lithium ion secondary batteries containing 1,3-propane sultone and/or 1,4-butane sultone in the electrolyte have been proposed as well (see PTL 5).